Modified clear_callback() to create empty  list for unknown key
Modified clear_callback() to create empty  list for unknown key

ejeschke/ginga,naojsoft/ginga,ejeschke/ginga,naojsoft/ginga,pllim/ginga,ejeschke/ginga,Cadair/ginga,rupak0577/ginga,Cadair/ginga,pllim/ginga,rupak0577/ginga,rajul/ginga,stscieisenhamer/ginga,naojsoft/ginga,rajul/ginga,rajul/ginga,pllim/ginga,rupak0577/ginga,stscieisenhamer/ginga,Cadair/ginga,stscieisenhamer/ginga